My kitchen opens directly to the dining room area, which has replacement flooring (1941 year built house) that is 2" wide solid red oak planks 1/2" thick nailed directly to the original 1" x 6" plank subfloor laid on the diagonal with 1/8" spaces between, over dirt floor crawl space (1 story house). The kitchen has vinyl sheet flooring over an older layer of sheet vinyl over particle board over old plywood over the 1 x 6 subfloor, which is continuous from DR to kitchen. There was a chunky threshold installed over the seam between the DR oak hardwood and the kitchen floor. I'm removing the kitchen floor down to the original 1 x 6s. I can have the same 1/2" solid oak hardwood installed in the kitchen to flow seamlessly from the DR, but wondered if there is a pre-finished product that I could use to avoid the mess of sanding and finishing in place. I don't mind some difference in the look between the two floors, but do not want a threshold or reducing strip so the two floors must be level. Does engineered flooring come in 1/2" thick and can it be installed directly to the 1 x 6 subfloor without any additional plywood layer? Thanks for any insights or ideas for product that can be installed directly to 1 x 6 subfloor.

Are you sure you have red oak? Is it possibly fir? Engineered flooring usually needs a "chunky transition" that sits on top of floor, to cover the seam and hold the floor down if glued. I believe, installed, sanded and finished floors are ultimately superior to any prefinished application. If this is a do it yourself job, or sanding is not in your budget, I would recommend 3/4" solid prefinished flooring with a beveled transition(pieces beveled to height and running perpendicular to existing floor) to meet existing flooring.

Just about everyone makes a 1/2in engineered. You can put a header in the doorway ( a strip of wood perpindicular to the other flooring direction) and run it off that. And yes you can nail it but it needs to be really flat. Check the manufacturers installation requirements for the correct fastener.